October 2020

Intermittent Fasting: What to Tell Patients

Fasting isn’t new. But in recent years, intermittent fasting has been growing in popularity. Many celebrities tout its benefits for a variety of reasons. What does the evidence say?

Intermittent fasting is a broad term. Alternate-day fasting, periodic fasting, and time-restricted feeding are all considered intermittent fasting.

  • Alternate-day fasting involves consuming under 25% of energy on fasting days, and then eating normally every other day
  • Periodic fasting involves restricting energy intake 1-6 days weekly – most commonly 2 days each week, and then eating normally for the rest of the week
  • Time-restricted feeding involves eating only during a certain time frame each day

Regardless of the type of intermittent fasting used, the end result is generally reduced calorie intake.

While intermittent fasting is increasingly promoted as a cure-all practice for many conditions, clinical research only shows benefits for people with obesity. There’s evidence that intermittent fasting, usually combined with calorie restriction on non-fasting days, can reduce weight by about 15% in overweight and obese people. But it’s important to note that it doesn’t seem to work any better than maintaining a continuous calorie-restrictive diet. Nor does it seem to increase weight loss in people who aren’t overweight. It’s been evaluated for a wide range of other conditions, including blood sugar control and heart disease, but there’s no strong evidence that it helps.

If patients are interested in intermittent fasting, tell them that there haven’t been any serious side effects reported when tried for up to one year. Some people might find it easier to maintain than a continuous calorie-restrictive diet, so it may be worth a try in patients who have struggled with other weight loss approaches. But remind patients that it’s important to meet nutritional needs during non-fasting periods. Also emphasize that intermittent fasting is very different from prolonged, water-only fasting. This should only be done under strict medical supervision. Prolonged fasts without medical supervision have resulted in death.

To learn more about fasting, check out our recently updated monograph.

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